1. Field of the Invention
The present invention relates to a gauge for determining, adjusting, and checking radiographic exposures, that will aid radiographers in producing x-ray films which are correctly exposed at various machine settings of voltage, current, and exposure time, and for various films, grids, screens and other accessories affecting exposure, or any other factors that effect exposure.
2. The Prior Art
For purposes of describing the device according to the present invention, we shall describe a phantom, or gauge, developed specifically for dental use. It is understood, however, that the gauge may be adapted for use with other parts of the body.
Much attention in recent years has been given to reducing the amount of x-rays dental patients receive during routine checkups. One way of reducing the amount of x-rays is to provide for the taking of better x-ray pictures, thus reducing the need for re-takes. In order to accomplish this goal, the x-ray machine must be calibrated frequently to ensure proper exposure in spite of possible drifting in supply voltage to the machine, or aging and temperature variations of the film developer.
The prior art tools that were developed depended on the judgement of the dentist to determine whether the film was correctly exposed.
Heretofore, patients, skeletal phantoms and step wedges had been used for testing film exposure. Using patients to test exposure has two major disadvantages, the first being that time consuming evaluation of anatomic detail is required and the second being that unnecessary patient exposure to potentially harmful x-rays is required. The skeletal phantoms also require time consuming evaluation of anatomic detail and the phantoms are costly.
The film step wedge is one type of prior art device used as a standard for the dentist to compare with his film.
The device includes a piece of thin metal having just the right thickness so that a film exposed through it correctly (x-ray voltage, current and time, as well as film speed and same position relative to the machine as the patient's tooth) would have exactly the same density as the center step of the film step wedge. If the density obtained were the same as either step of the film wedge next to the center, the dental film exposed that way would be a bit too dark (or too light) but still usable. If it matched the steps further from the center, towards either end of the film wedge, the film would be too dark or too light to be useful clinically.
However, this prior art film step wedge, which was made on gray base film, does not work adequately on all brands of film. Each brand comes with variations to suit each user's taste. One such variation is the color of the plastic base on which the emulsion is put. A film wedge made on gray base film can not be used to test blue base film because the densities can not be easily compared by eye. Even with the same density, the films will look different.
The step wedge provides a number of other disadvantages. When visual comparisons are made, the amount of correction cannot easily be established. Further, a correctly exposed reference film is required. Finally, there is no way to distinguish between the upper limit of correct exposure and overexposure and between the lower limit of correct exposure and underexposure.
Densitometric comparison of the step wedge films removes the subjective judgement but still requires a correctly exposed reference film and the densitometer is a costly piece of equipment.
Thus a different tool was developed which included a brass step wedge. This has five brass steps, of thickness 0.004, 0.006, 0.008, 0.010 and 0.012 inch. Additionally, in another spot, a sheet of brass 0.008 of an inch thick is included. The user, when the x-ray machine is in perfect operating adjustment, and has fresh developer, etc., (such as when a repairman has placed it in good working order) exposes a film on the wedge side of the phantom, or tool and develops it. He inserts the film into a slot in the phantom, and leaves it there. When he wants to check working conditions, he exposes another film through the 0.008 inch brass plate side of the phantom and compares it with the wedge. If he decides to change to a different film, he makes another film wedge.
Both of these prior art phantoms may not be accurate or consistent in the results achieved because they depend on the subjective judgement of the dentist to determine whether the exposure is as it should be.
An "X-ray Checker", another type of prior art phantom, is described in the Textbook of Dental Radiography, by O. E. Langland and F. H. Sippy (Illinois: Charles C. Thomas 1973) at pages 191-193. However, the disadvantages discussed with respect to the step wedge likewise occur with the "X-ray Checker".